Aromatic polyimides possess physical properties which makes them particularly useful in microelectronic applications. The properties include high thermal stability, high insulating properties - low dielectric constant, excellent adhesion to substrates, high radiation stability, excellent planarization and high flexibility in comparison to inorganic dielectrics. Various efforts to utilitize polyimides in microelectronic applications are reported in the literature which can be generally classified as complicated and not entirely satisfactory.
Rubner et al, U.S. Pat. No. Re. 30,186 discloses a method of preparing a negative photoresist by preparing a mixed diester - diacid chloride of pyromellitic dianhydride containing an ethylenically unsaturated group in the ester portion. This derivative is then reacted with an aromatic diamine to yield a poly(amide-ester) which is mixed with a photoinitiator and solvent to form a resist solution which is coated on a substrate, exposed and developed in the conventional manner. After development the relief structure is heated to drive off the crosslinked ester portion and form the polyimide. The procedure is complicated; requiring long, low temperature reaction times in excess of a day just to form the poly(amide-ester) precursor.
U.S. Pat. No. 4,565,767 discloses a light sensitive composition consisting of an aromatic polyamic acid, a bisazide and an ethylenically unsaturated tertiary amine. The storage stability of polyamic acid precurssor is known to be poor.
U.S. Pat. No. 4,677,186 discloses a light sensitive polymer from an aminodicarboxylic acid. This polymer is light sensitive and crosslinkable by radiation to form a negative resist.
EP No. 141,781 discloses a negative photoresist consisting of a polymethyl substituted, solvent soluble polyimide and a diazide which is crosslinkable by imagewise exposure to actinic radiation in the exposed area and developed by conventional solvent techniques. The methyl substituent of this polyimide provide for its solubility but they give rise to poor oxidative thermal stability.
The literature takes the general approach of preparing light sensitive negative photoresists from either a polyamic acid, polyimides with a high percentage of lower alkyl ring substituents or a photosensitive polymer. These approaches involve complicated synthesis methods and the polyamic acid based compositions have storage stability problems. It is the object of this invention to provide negative photoresist compositions by simplified and cost efficient procedures that possess good storage stability and good photoresist properties.
The photosensitive polyimide compositions of the invention can have additional applications in other areas; for example in the production of laminating varnishes for the manufacture of high temperature laminates or composites; the manufacture of formed or molded parts having a defined physical shape or in other electronic applications such as liquid crystal displays. These applications and others will be readily apparent to the worker of ordinary skill in light of the foregoing disclosure.